A pyrophosphate, particularly melamine pyrophosphate is a compound obtained by bonding between pyrophosphoric acid, which is a polyphosphoric acid, and melamine and is useful as a flame retardant added to paint, synthetic resins, and the like. Various processes for producing melamine pyrophosphate have been so far proposed.
For example, Patent Document 1 below discloses a process comprising mixing melamine and hydrochloric acid in an aqueous solution to make melamine hydrochloride and adding thereto sodium pyrophosphate to precipitate melamine pyrophosphate. However, this process uses an expensive pyrophosphate as a raw material and involves a washing step and a filtration step to remove the halogen and therefore has the problem of high production cost.
Patent Document 2 below teaches preparing melamine pyrophosphate by causing pyrophosphoric acid and melamine to react with each other at 0° to 60° C. in an aqueous solution. This process, too, has the problem of high production cost on account of the use of expensive pyrophosphoric acid and the involvement of a filtration step.
Production of melamine pyrophosphate or melamine polyphosphate by firing melamine orthophosphate to induce dehydration condensation in a solid phase is known as disclosed in Patent Documents 3, 4, and 5 below.
However, when a pyrophosphate such as melamine pyrophosphate is produced by firing melamine orthophosphate in a solid phase to cause dehydration condensation, an unreacted material or a product can adhere to the inner wall of firing equipment or the stirring mechanism such as a stirring blade. This is because of the influences of by-produced water and viscous overreaction products, such as a triphosphate and higher polyphosphates. These substances form a persistent deposit on the inner wall of the firing equipment, the stirring blade, and the like, which hinders uniform heating and firing, and the adhesion loss leads to reduction in yield.